Not for LUNs, only for images.
The available formats in 4.3 are documented here:
incremental means you checked the checkbox "Enable incremental backup" when creating a disk.
But note that the fact that we will create qcow2 image is implementation detail and the behavior
may change in the future. For example, qemu is expected to provide a way to do incremental
backup with raw volumes, and in this case we may create a raw volume instead of qcow2 volume.
(actually raw data volume and qcow2 metadata volume).
If you want to control the disk format, the only way is via the REST API or SDK, where you can
specify the format instead of allocation policy. However even if you specify the format in the SDK
the system may chose to change the format when copying the disk to another storage type. For
example if you copy qcow2/sparse image from block storage to file storage the system will create
a raw/sparse image.
If you desire to control the format both from the UI and REST API/SDK and ensure that the system
will never change the selected format please file a bug explaining the use case.